By now we know that Paul McCartney did indeed front a Nirvana “reunion” at last night’s 12-12-12: Concert for Sandy Relief, as had been reported. We also know Nirvana bassist Krist Novoselic wasn’t entirely joking when he tweeted that their performance would debut a new song mixing “Helter Skelter” and “Scentless Apprentice,” though he didtweet at the Pope. That song, “Cut Me Some Slack,” featured Novoselic, drummer Dave Grohl, and Nirvana touring guitarist Pat Smear, and “it was indeed heavy,” as our own Chris Martins and Kory Grow reported. “It freaking slayed, in fact, and once it was played, nothing else mattered.”

Well, Grohl must be hoping something else matters, because he also revealed last night the song will appear on the soundtrack to Grohl’s directing debut Sound City, the recording-studio documentary he’s premiering at Sundance Film Festival next month. That above is a promotional video including a roaring snippet of “Cut Me Some Slack.” The soundtrack album will be titled Sound City – Real to Reel, and it will include “music from and inspired by” Grohl’s movie. No release date or track list yet.

In the meantime, watch McCartney, Grohl, Novoselic, and Smear’s performance last night at New York’s Madison Square Garden below, plus clips from some of the night’s many other stars, including Bruce Springsteen and the E Street Band, Michael Stipe and Chris Martin, Kanye West, and the Rolling Stones.